Methods are known to the art in which nitrogen compunds and other compounds which will decompose to form ozone-decomposing substances are fed to the vicinity of the welding or working arc. Attention is drawn in this respect, by way of example, to Australian Pat. No. 267,217, German Pat. Nos. 1 016 865, 1 039 675 and 496 337 and to U.S. Pat. Nos. 313,348, 2,576,783, and 3,243,569.
For example, Australian Patent Specification discloses an improvement in gas shielded arc-welding processes in which the shielding gas contains a minor percentage of nitrous oxide (N.sub.2 O). The overall percentage of N.sub.2 O is governed by the amounts of denitriding and/or deoxidizing and/or fluxing materials used, and is stated to be from 0.5% to 20% by volume of the shielding gas. Although such quantities will be sufficient to reduce ozone in the immediate vicinity of the welding arc, they are harmful to the weld produced. For example, quantities of N.sub.2 O in excess of 0.1% when TIG welding will cause the electrode to burn up, whilst quantities of more than 0.5% in MIG welding will result in a poor weld.
The compounds described in the aforementioned specifications are not intended for the express purpose of decomposing ozone in the welding area. In order to be truly effective they must be fed into the arc itself and then converted into purely nitric oxide.
Experiments have shown that the production of ozone reaches a peak within a few thousandths of a second after the arc has been lit, and may even be present in the welding area before the arc is lit. Thus, before the said compounds have decomposed to form a substance which will react directly with ozone, the welder has been subjected to excessive amounts. In addition, many of the compounds known to be introduced to the welding arc for one purpose or the other, but not for the purpose of reducing ozone, produce gases which are far more harmful than ozone itself.
One object of the invention is to provide a method when working with an electric arc by which ozone can be effectively decomposed in the welding area without deleteriously affecting the weld of, when cutting, the cut surfaces.
In the method of the invention there is introduced into the welding or working area a nitrogen compound which is preferably nitric oxide, but which in certain cases may also be nitrous oxide or ammonia (NH.sub.3). If added in limited amounts, it had been found that N.sub.2 O and NH.sub.3 will assist in decomposing O.sub.3 to acceptable limits without affecting the weld. By welding area is meant here an area extending from the immediate vicinity of the arc to a location removed some distance therefrom.
Conveniently, the nitric oxide is introduced into the welding area at a rate not exceeding 100 ml/min., and preferably betweem 1 ml/min-30 ml/min. Although the nitric oxide will react with the ozone to form nitrogen dioxide (NO.sub.2), which is in itself a pollutant, this can be accepted, because whilst the international threshold limit for O.sub.3 is as low as 0.1 ppm (0.2 mg/m.sup.3) the corresponding limit for NO.sub.2 is as high as 5 ppm (9 mg/m.sup.3).
The nitric oxide may be introduced into the welding or working area in a pure form, or together with a shielding gas. For example, the NO may be mixed with the shielding gas within the welding torch itself, or in the gas line leading thereto, or may be charged to said area in a pure form and there become mixed with the gas by convection currents. By passing a part of the shielding gas containing nitric oxide to a location in the welding or working area remote from the vicinity of the arc, the additional safeguard is obtained whereby any residual ozone, which by chance has not reacted, will come into contact with further amounts of nitric oxide, thereby to be decomposed.
Nitrous oxide is introduced into the welding or working area in mixture with a shielding gas, said nitrous oxide being present in an amount not exceeding 0.45% of the gas. Whilst amounts of this magnitude do not deleteriously affect the workpiece or the electrode, they are effective to reduce the amount of ozone in the working area to a safe level.
When the welding or working operation is carried out in the presence of a shielding or protective gas, the gas may contain at least two of the aforementioned NO, N.sub.2 O or NH.sub.3 in mixture, although the amount of N.sub.2 O must not exceed 0.45% of the gas.
Decomposition of ozone in the vicinity of the arc can also be accomplished by coating the electrode and/or the workpiece with a nitrate, such as ammonium nitrate NH.sub.4 NO.sub.3.
The method of reducing or even eliminating the formation of ozone in accordance with the invention can be carried out with various means, depending upon the welding or other operating method being used, or in other circumstances connected with the welding or other operating process. Various ways of carrying out the method of the invention will now be described with reference to examples which further illustrate the invention.
As will be apparent from the brief description of the invention given above, the method of reducing the formation of ozone according to the invention does not suffer from the disadvantages encountered with hitherto known suction and evacuating methods.
The invention will now be further described in more detail with reference to apparatus for carrying out the method in which a shielding gas is used.